Circuit for canceling oscillating in public address systems



Get. 1, 1963 T. P. MILLER E' Al CIRCUIT FR Emmi; OSCILLATING 1N PUBLCADDRESS SYSIEIS Filed Sept. 12, 1960 INVENTORS.

BY laNfER 3,105,877 CmCUT FR CANCELWG OSCHILATING IN PUBiLiC ADDRESSSYSTEMS Thomas P. Miller, Mount Prospect, and Solly L. Futlaley andRaymond N. Reinertson, Chicago, 1li., assignors to InternationalTelephone and Telegraph Corporation, New York, N.Y., a corporation ofMaryland Filed Sept. 12, 1960, Ser. No. 55,396 8 Claims. (Cl. 179-1)This invention relates to public address systems and more particularlyto circuits for canceling the oscillations sometimes caused byacoustical coupling between the input and output of such public systems.

A public address system is very often used to amplify and reinforce thevoice of a person who is speaking into a microphone placed withinacoustical range of an associated loud speaker that is broadcasting thesound of the voice. When the equipment is used under these conditions,the sound emanating from the loud speaker is fed back into themicrophone vas noise which is reampliiied to cause oscillation. Thisfedback sound is commonly called acoustical coupling.

ln the past, systems for preventing oscillations caused by acousticalcoupling have been unduly complex and have failed when used improperly.More particularly, in some systems it is necessary to cut-off or reducethe volume of nearby loud speakers each time that an adjacent microphoneis used, thus requiring constant attention on the part of the user.Other systems use either directional or differential microphones toavoid acoustical coupling, thus preventing persons using the system formoving about freely. Finally, in these and other systems, the problemsare only minimized because the system may oscillate if improperly used,as when a directional microphone is placed directly in front of a loudspeaker.

Accordingly, an object of this invention fis to provide new and improvedpublic address systems, and more parlticularly to provide public addresssystems having circuits for canceling the effects of acoustical couplingbetween the input and output thereof.

A further object of this invention is to develop a control signal fromthe voice currents appearing in a public address system forautomatically canceling the effects of acoustical coupling between themicrophones and the loud speakers used in the system.

A specific object of this invention is to provide a new and improvedcircuit for mixing two audio frequency signais. A more particular objectis to cancel a band of frequencies comprising the mixed signals whiletransmitting a band of yfrequencies adjacent thereto, with thetransmitted band having substantially full 'power up to the canceledband and a sharp cut-olf thereafter.

ln accordance with one .aspect of this invention, the public addresssystem microphones and loud speakers are interconnected by means of aprincipal voice channel having an amplifier therein. Coupled to theprincipal channel and interposed between the microphones and ampliiieris a supplementary channel adapted :to bleed-off a high frequencyportion of the voice signals appearing in the principal channel. Thefrequencies of this bledolf signal are the same as the resonantfrequencies of the public address system. The bled-olf signals are thenphase shifted by approximately 180 and fed into a mixer circuit whichcombines the original voice signals and the phase shifted signals witheach other to eliminate a band of frequencies which is the same as theresonant frequencies of the system. Thus, acoustical coupling betweenthe microphones #and loud speakers is at non-resonant frequencies andthe system does not oscillate.

The above mentioned and other features and objects of this invention andthe manner of obtaining them will become more apparent and the inventionitself will be best understood, by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. l shows by block diagram a public address system constructed'inaccordance with this invention; and

FIG. 2 shows by schematic circuit diagram, the circuitry used tocomplete the hollow blocks of FIG. 1.

Where possible, simple terms are used md specific items are describedhereinafter to facilitate as understanding of the invention; however, itshould be understood that the use of such terms and references to suchitems are not to ybe construed as a disclaimer of the full range ofequivalents normally given in paten-t lafw. For example, varioussemiconductor devices fare shown as PNP junction type devices; however,other electronic devices may be used also. Reference is made to abandpass filter which passes frequencies in the voice range fallingabove 260() cycles per second. Other frequency ranges may also beaccommodated. The bandpass filter is here shown as a couplingtransformer tuned by a parallel capacitor while other filter circuitsmay be used also. Quite obviously, other examples could be selected toillustrate hofw the devices shown and described are entitled to a widerange of equivalents.

Briefly, the principal components of the publ-ic address system shown inFIG. l include a pair of microphones 1li, 11 connected to a mixercircuit 12 by Way of a principal voice channel 13. Coupled to theprincipal channel 13 is a supplementary channel 14 which bleeds-oli" aportion of the voice signal having a relatively narrow band width offrequencies that pass through a bandpass filter 1S. The bled-olf signalis then fed through phase shifter 16, which shifts the bled-off signalby approximately This phase shifted signal is applied through the mixer12 where it is combined with and, therefore, subtracted from theoriginal voice s-ignal. The effect is to eliminate the `band offrequencies at which the system is resonant. With this arrangement, thesound amplified at 17 tand thereafter broadcast from la loud speaker 18does not include the frequencies which may cause the system to oscillatewhen fed back from the loud speaker into the microphones.

For a more complete understanding of the invention, referenceis made tothe schematic diagram of FIG. 2. More particularly, the connectionbetween the microphones and the output amplifier circuit 17 is completedfrom the principal channel 13 through a coupling transformer Zit. rl'heresistors 21-23 match the impedance of the coupling transformer 20 tolthe impedance of the voice channel.

To bleed-oli a portion of the voice frequency lsignals originating atthe microphones, the supplementary voice channel 14 is connected betweenchannel 13 and the primary winding of a transformer 25 in the bandpassfilter circuit 15. Preferably, this transformer has a relatively highimpedance to limit the current flowing from the channel 13 to the phaseshifter 16. The transformer 25 is tuned by an adjustable capacitor 25ato pass a band of frequencies which coincides with system resonance. Inone system, there were satisfactory results when the capacitor wasadjusted to tune transformer 25 to pass all frequencies appearing inchannel 13, which were higher than 2600 cycles per second.

Means are provided for shifting the phase of the signals bled-off theprincipal voice channel and passed through the bandpass filter 15. Moreparticularly, the phase shifter 16 includes, as principal components, aphase shifter network V26 of any conventional design and a pair ofamplifiers 2S, Z9, all of which are interconnected by a number ofsuitable coupling transformers. The phase shifter provides a controlsignal which is combined with the original voice signal in mixer circuit12 to cancel the noise band. The amplifiers bring the phase shiftedsignal to a predetermined signal strength which is then appiied to themixer circuit 12.

The amplifier circuit 23 includes a PNP transistor arranged in a commonemitter configuration. A base biasing potential is applied to transistor28 by a voltage divider including a resistor 3ft and the second-arywinding of a coupling transformer 31, the voltage divider beingconnected between battery and ground. The collector load of transistor28 is completed to battery by way of the primary winding of a thirdcoupling transformer 35 and a load dropping resistor 36. The base toemitter bias is provided by the impedance of the secondary winding 31. Adecoupling capacitor 37 is connected between the primary winding oftransformer 35 and ground.

In this circuit, the coupling transformer 35 is a high impedance deviceprovided so that other voice channels may be connected in parallel anddriven from a common phase shifter circuit, such as 16. T o match thehigh imedance of transformer 35 to the relatively low impedance of thesecond amplifier '29, a second coupling transformer 4t) is connected tothe secondary winding of transformer 35. A loading resistor 41 isconnected in parallel across both the secondary winding of transformer35 and the primary winding of transformer 46 to provide impedancematching. lf there is no need to provide other parallel voice channels,both of the transformers 35, 40 may be omitted.

To provide a fine adjustment in the phase angle of the bled-off signal,a network including a capacitor 42 is connected in parallel and a pairof resistors 43, 44 are connected in series with the secondary windingof coupling transformer 40. As well known to those skilled in the art,current passing through a capacitor leads the voltage by a 90 phaseangle. `Current passing through a linear resistor is in phase with thevoltage. Therefore, the resultant impedance of circuit 42-44 has a phaseangle between and 90. The exact angle is determined by the selection ofthe circuit components.

The amplifier circuit 29 includes a PNP transistor connected in a commonemitter configuration. The base bias for transistor 29 is provided by avoltage dividing network which may be traced from ground through a gaincontrol potentiometer 46, a resistor 47, the primary Winding of a fourthcoupling transformer t), and a voltage dropping resistor 51 to battery.yConnected to the emitter is a bypass and decoupling capacitor 52. Theresistor 53 is avoltage dropping and loading device for matching theimpedance of coupling transformer 50 to the impedance of the input tomixer circuit 12.

vMeans are provided for mining the voice signals occurring in theprincipal channel 13 and the phase shifted signals received from thephase shifter 16 to cancel the band of frequencies which coincide withthe resonant frequencies of the public address system. Moreparticularly,

the circuit for accomplishing this function includes a pair of PNPtransistors 6i), 61 each of which is connected in a common emitterconfiguration.

The base bias for transistor 6ft is provided by the voltage dividerincluding the series connected resistors 62, 63 connected betweenbattery and ground while the base bias of transistor A61 is provided bythe series circuit traced from battery through resistors v64, 65, andthe secondary winding of transformer Z@ to ground. The emitter oftransistor is biased by the voltage drop across resistor 53 and by thephase shifted signal applied via the coupling transformer 50. Thecollector load on transistor 60 results from the internal resistance oftransistor 61. The collector to base bias on transistor 61 results fromthe voltage drop across resistor 65, while the base to emitter bias fortransistor 61 is provided by the internal resistance of transistor 6&1.

4 The voice signals appearing in the principal channel 13 are appliedacross the coupling transformer 20 to the base or control electrode b oftransistor 61 and the phase shifted signal is applied to the emitterelectrode e of tranV sistor 69. The output or collcetor electrode c oftransistor 61 is coupled to the base or input electrode b of transistor6ft via a feedback capacitor 66 and the output or collector electrode cof transistor 69 is connected to the emitter electrode e of transistor61.

The operation of the mixer circuit 12 is as follows: The original voicesignals are applied across the inductive coupling of transformer 2t? tothe base electrode b of transistor 61 and the phase shifted signals areapplied to the emitter electrode e of transistor 60. The lowertransistor 6i) functions as a variable resistor connected in the biasingcircuit of the emitter electrode e of transistor 61. More specifically,as the current of the phase shifted control signal rises and falls thecurrent flowing through the emitter-collector circuit of transistor 61also rises and falls. Since the signals applied to base b and emitter eare out of phase, transistor 60 tends to cut down the emitter current oftransistor 61 at the same time that the signal applied from channel 13to its base b tends to raise the emitter current, and vice versa. Thecircuit values are selected so that cur-rent flowing through theemitter-collector of transistor 61 tends to remain unefected by inputsignals applied to its base throughoutthe entire frequency range of thephase shifted signals applied from phase shifter 16 through transistor60 to the emitter of transistor 61. in other words, the frequencies ofthe phase shifted signal are eliminated from the signals passing fromchannel 13 through the mixer circuit 12 to the loud speakers.

Means are provided for giving a sharp cut-off at the upper limits of thefrequency band passed through tram;

sistor 61. More specifically, with the control described thus far, thereis a tendency for the phase shifted control signal not only to eliminatea band of frequencies, but also to attenuate signals of lower frequencyin the band passed through transistor 61. Therefore, the output orcollector electrode of transistor 61 is coupled to the input or baseelectrode of transistor 6i) via a coupling capacitor 66. The capacitor66 is selected to pass frequencies in the attenuated band and below theeliminated band. Thus, in the frequency band passed by capacitor 66,'thebase bias ofV transistor 60 is raised or lowered to counteract theeffects produced on the emitter-collector current by the phase shiftedsignalsv applied through networkY 16.

Preferably, the signal applied through capacitor 66 to the baseelectrode b of transistor 60 has less effect upon its emitter-collectorcurrent fiow than the signal applied to its emitter from phase shifter16. Thus, any tendency for the cut-ofi` point of the eliminated band toshift will vbe in favor of a cancelation of the noise frequencies thatkcause circuit oscillations. Hence, the upper limit of fre'4 quenciespassed through amplifier 61 are given a sharp cutoff without appreciableattenuation of lower than cut-.y

The resistor 72 and the primary Winding of transformer 76 provide theload for the collector circuit of transistor '71. The emitter bias isprovided by the voltage drop acrossresistor 75. 'A decoupling capacitor77 provides a bypass to ground. q is a coupling transformer 76 forYapplying the amplified voice signals to one or moreloudspeakers viaconductors 79. The tuning of capacitor'78 determines the frequencyresponse of the amplifier output.

In one circuit constructed lin accordance with this invention, voicesignals originating atY microphones 10,11

In the output circuit of transistor 71' l and applied over principalchannel 13 had a frequency range of approximately 200 to 3000 cycles persecond. In this exemplary public address system, the resonant frequencyrange extended from about 2600 to 3000 cycles per second. Therefore, afrequency band of 2600 t 3000 cycles per second, bled-orf insupplementary channel 14, was applied through the phase shifter circuit16 to the mixer circuit 12. In the mixer circuit 12, the original Voicesignals appearing in channel 13 and the phase shifted signals fedthrough circuit 16 were combined with each other to cancel outputsignals in the resonant band. With acoustical coupling between theloudspeakers and the microphone, the system did not break intooscillation when a loudspeaker having a -watt audio output waspositioned immediately above and behind the head of a person speakinginto the microphone.

While the principles of the invention have been described above inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of the invention.

We claim:

l1. A public address system having a resonant frequency band width,means for transmitting voice frequency signals through said system, saidsignals including frequency falling in said resonant band width, meansin said system for canceling that portion of the signals which includesfrequencies falling in said band width, said canceling means including apair of semiconductor devices, one of said devices having an inputelectrode coupled to be energized by said voice signals and an outputelectrode coupled to an input electrode of the other of said devices, anoutput electrode of said other device being connected to control thebias on a third electrode of -said one device, means for applying atleast a portion of said voice signals phase shifted to a third electrodeof said other device, and means also coupled to the output electrode ofsaid one device for broadcasting sounds corresponding to said voicefrequencies less said canceled band Width.

2. A public address system having a resonant frequency band width, meansfor transmitting voice frequency signals through said system, saidsignals including frequency falling in said resonant band width, meansin said system for canceling that portion of the signals which includesfrequencies falling in said band width, said last named means includinga pair of scmicoductor devices, one of said devices having an inputelectrode coupled to be energized by said voice signals and an outputelectrode coupled to an input electrode of the other of said devices viaa capacitor, said capacitor being tuned to pass frequencies below theupper limit of said band width, an output electrode of said other devicebeing connected to control the bias on a third electrode of said onedevice, means for applying a portion of said voice signals falling insaid band width, phase shifted to a third electrode of said otherdevice, and means also coupled to the output electrode of said onedevice for broadcasting sounds corresponding to said voice frequenciesless said canceled band width.

t3. A circuit for preventing oscillation responsive to acousticalcoupling in a public address system comprising at least a microphone anda loudspeaker interconnected by an amplifier, there being acousticalcoupling between said loudspeaker and said microphone, means interposedbetween said microphone and said amplifier for bleedingoff a portion ofthe voice signals transmitted from said microphone to said amplifier,means for shifting the phase of said bled-off signal, means including amixer circuit for combining said phase shifted signals and said voicesignals, said mixer circuit comprising a pair of transistors, one ofsaid transistors having its input electrode coupled to said microphoneand an output electrode coupled to an input electrode of the other ofsaid transistors, an output electrode of said other transistor beingconnected to control the bias on a third electrode of said onetransistor,

means for applying said phase shifted signal to a third electrode ofsaid other transistor, and means for driving said loudspeaker by saidcombined signal emanating from said mixer circuit.

4. A circuit for preventing oscillation responsive to Iacousticalcouplings in a public address system comprising =a principal voicechannel for carrying voice frequency signals, means including anamplifier in said principal voice channel for amplifying signalsappearing therein, a loud speaker coupled to be driven by said amplifiedsignals, la supplementary voice channel coupled to said principal voicechannel for bleeding-olf a portion of the signals appearing therein,bandpass filter means in said supplementary channel for passing a bandof frequencies which coincides with the resonant frequencies of thepublic address system, Kmeans for shifting the phase of said voicesignals passed thro-ugh said bandpass filter means, Kand a mixer circuitcomprising a pair of transistors, one of said transistors having aninput electrode coupled to said principal voice channel and an outputelectrode coupled to an input electrode of the other of saidtransistors, -an output electrode of said other transistor beingconnected to control the bias on a third electrode of said onetransistor, iand means for applying said phase shifted signal to -athir-d electrode of said other transistor.

5. A circuit for preventing oscillation responsive to acousticalcouplings in la public address system comprising -a principal voicechannel for carrying voice frequency signals, means including an'iamplifier in said principal v-oice channel for amplifying signalsappearing therein, a loud speaker coupled to be drive by said amplifiedsignals, la supplementary voice channel coupled to said principal v-oicechannel for bleeding-off la portion of the signals appearing therein,bandpass filter means in said supplementary channel for passing a bandof frequencies which coincides with the resonant frequencies of thepublic address system, means for shifting the phase of said voicesignals passed through said bandpass filter means, and a mixer circuitcomprising a pair of transistors, one of -said transistors having aninput electrode coupled to said principal voice channel Kand an outputelectrode coupled via a capacitor to an input electrode of the other offsaid transistors, said capacitor being tuned to pass frequencies belowthe upper limit of said band Y of frequencies, an output electrode 'ofsaid other transistor being connected to control the bias on' la thirdelectrode of said one transistor, `and means for applying said phaseshifted signal to a third electrode of said other transistor.

6. A mixer circuit for combining two electrical signals comprising `apair of transistors, one of said transistors having lan input electrodecoupled to be energized by one of said signals and an output electrodecoupled to an input electrode of the other of said transistors, anoutput electro-de of said other transistor being connected to controlthe bias on a third electrode of said one transistor, means for applyingthe other of said signals to `a third electrode of said othertransistor, and an output circuit coupled to the output electrode ofsaid one transistor.

7. In -a mixer circuit for combining two electrical signals utilizingmeans for phase shifting one of said signals relative to the other, thecombination therewith comnals, one of said signals including a narrowband of fres quencies, Vthe other of said signals having a wide bandWidth which includes said narrow band width, the combination comprisinga pair of transistors, one of said transistors having an input electrodecoupled to he energized 'by saidk other signal :and its output electrodecoupled via a capacitor to an input electrode of the other of saidtransistors, said capacitor bein-g tuned to pass frequencies in saidwide band width immediately outside of said narrow band, an outputelectrode of said other transistor being connected rto control the biason a third electrode of said one transistor, means for applying saidnarrow band width in phase shifted relation to said wide band to a thirdelectrode of said other transistor, tand an output circuit means coupledto the output electrode of said one transistor.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Van N ostrand, The International Dictionary of Physics Y andElectronics (pages 133-134 relied on).

1. A PUBLIC ADDRESS SYSTEM HAVING A RESONANT FREQUENCY BAND WIDTH, MEANSFOR TRANSMITTING VOICE FREQUENCY SIGNALS THROUGH SAID SYSTEM, SAIDSIGNALS INCLUDING FREQUENCY FALLING IN SAID RESONANT BAND WIDTH, MEANSIN SAID SYSTEM FOR CANCELING THAT PORTION OF THE SIGNALS WHICH INCLUDESFREQUENCIES FALLING IN SAID BAND WIDTH, SAID CANCELING MEANS INCLUDING APAIR OF SEMICONDUCTOR DEVICES, ONE OF SAID DEVICES HAVING AN INPUTELECTRODE COUPLED TO BE ENERGIZED BY SAID VOICE SIGNALS AND AN OUTPUTELECTRODE COUPLED TO AN INPUT ELECTRODE OF THE OTHER OF SAID DEVICES, ANOUTPUT ELECTRODE OF SAID OTHER DEVICE BEING CONNECTED TO CONTROL THEBIAS ON A THIRD ELECTRODE OF SAID ONE DEVICE, MEANS FOR APPLYING ATLEAST A PORTION OF SAID VOICE SIGNALS PHASE SHIFTED TO A THIRD ELECTRODEOF SAID OTHER DEVICE, AND MEANS ALSO COUPLED TO THE OUTPUT ELECTRODE OFSAID ONE DEVICE FOR BROADCASTING SOUNDS CORRESPONDING TO SAID VOICEFREQUENCIES LESS SAID CANCELED BAND WIDTH.
 6. A MIXER CIRCUIT FORCOMBINING TWO ELECTRICAL SIGNALS COMPRISING A PAIR OF TRANSISTORS, ONEOF SAID TRANSISTORS HAVING AN INPUT ELECTRODE COUPLED TO BE ENERGIZED BYONE OF SAID SIGNALS AND AN OUTPUT ELECTRODE COUPLED TO AN INPUTELECTRODE OF THE OTHER OF SAID TRANSISTORS, AN OUTPUT ELECTRODE OF SAIDOTHER TRANSISTOR BEING CONNECTED TO CONTROL THE BIAS ON A THIRDELECTRODE OF SAID ONE TRANSISTOR, MEANS FOR APPLYING THE OTHER OF SAIDSIGNALS TO A THIRD ELECTRODE OF SAID OTHER TRANSISTOR, AND AN OUTPUTCIRCUIT COUPLED TO THE OUTPUT ELECTRODE OF SAID ONE TRANSISTOR.